1. Field of the Invention
The present invention relates to an alternating-current (AC) motor control apparatus and an AC motor control method for performing torque control or speed control of an AC motor without using a position or speed sensor.
2. Description of Related Art
Methods for estimating the position and speed of an AC motor without using a position or speed sensor are roughly classified into methods in which the position and speed of an AC motor are estimated in accordance with a detected or estimated value of a motor induced voltage and methods in which the position and speed of an AC motor are estimated, by applying a high-frequency signal to the AC motor, in accordance with a detected value of a voltage or a current that depends on the inductance characteristic of the AC motor. The former method is suitable for driving an AC motor for which the inductance characteristic of the AC motor is not available in advance. However, in the former method, in a case where the frequency at which the AC motor is driven is low, since the induced voltage is low, the signal-to-noise (S/N) ratio is reduced due to the influences of measured noise and the nonlinearity of characteristics of a driving circuit. Hence, a speed estimation error is increased.
For example, WO2002/091558 suggests a technique in which the speed of a motor is estimated, not directly in accordance with an induced voltage, but by estimating magnetic flux in accordance with a motor model, and at the same time, by estimating an error signal in accordance with an estimated value of magnetic flux and a deviation between a redundant estimated value of a current and a detected value of a current, using a proportional-plus-integral compensator that reduces the error signal to zero.
In addition, Japanese Unexamined Patent Application Publication No. 2003-319697 suggests a technique in which a gain computing unit is improved in such a manner that a gain of a deviation amplifier used for correcting the input of a motor model is properly output and the accuracy and responsiveness of speed estimation are thus improved while the reliability and responsiveness of speed estimation are taken into consideration.